This invention relates to a booster seat of the type that enables a child to use an adult vehicle seat belt. The design of the booster seat provides enhanced performance in the event of an accident. In a frontal impact on the vehicle, the inertia load of the booster seat is applied directly to the lap strap instead of being applied to the lap strap via the child's body. Also, in the event of a side impact, lateral displacement of the booster seat relative to the anchoring points of the adult vehicle seat belt is resisted by the lap strap portion of the seat belt.